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Talking Chop Baseball Analysis Primer: Aging

MLB: NLDS-Workouts Robert Hanashiro-USA TODAY Sports

Living things age, and baseball players are no exception. Unfortunately, the details of aging in a baseball context have not been steady over time. A homer has always been a homer, but a 27-year-old’s potential has differed over baseball’s long history. That tends to lead to some confusion and consternation.

There was a point, where baseball was perhaps a little less deeply analyzed, that players got called up, and sort of floundered around while they gained experience. If good players got called up their early 20s, the general idea was that they’d figure out how to succeed in the majors by their late 20s, before aging made them unable to keep up with their younger peers, until they had lost so many steps, bat speed, or ticks on their fastball to be worth rostering at all. Under this premise (which was not just a premise, but an observed phenomenon since World War II through the late 2000s), players were fairly weak performers if called up around 21-22, continued improving through age 26 or so, then had peak performance (all of this is on average, every player is different!) from 26 through 28, and then declined fairly steadily. The result of this was an upside down U-shaped curve, where, relative to his peak, a player would be just as bad at age 21 as he was at age 35.

Since then, though, things have changed, and some of the best evidence originates from this Jeff Zimmerman piece at Fangraphs: That article features the following chart:

You can see that all lines but the green line look pretty similar, and have that upside-down U shape. But, the green line reflects data for the most recent group of hitters, and is instead like a warped hockey stick: it’s flat, and then features the same age-related decline curve. But, as the hyperlink to it says, the big finding is that hitters no longer peak, only decline. That doesn’t mean they start declining right away; the decline doesn’t get notable until around age 29. And again, this isn’t some kind of hard-and-fast rule for every hitter, it just describes what happens on average across the whole population. But, it’s a substantial deviation from the way things used to be.

So, when someone tells you, “We should get this hitter, he’s 26 and is going to be in his prime for the remainder of his team control years,” just remember -- that may have been valid in the past, but it’s not the past any longer.

Pitcher aging curves have also been researched and summarized by Zimmerman (and Bill Petti), here: and here: These articles (like everything linked in this primer) are well worth reading in full, but the striking thing is that the pitcher aging curves here are much more in line with the “new” hitter aging curves than the old ones. The general idea, again, is that pitchers tend to be relatively consistent from call-up to around 29 or 30, and then start to decline. Unlike hitters, there are some competing effects: pitcher walk rates actually do improve as pitchers get more experience, but velocity starts falling off almost immediately. The tension between these two things prevents there from being an overall effect on run prevention, but eventually the velocity decrease becomes too great to overcome, and walk rates go up as well, accelerating the decline.

It’s also worth noting that reliever aging curves are just a mess. I don’t mean intuitive, I mean depressing. According to the Zimmerman and Petti work, relievers actually maintain velocity after call-up a little better than starters, but their walk rate starts getting worse almost immediately. As a result, relievers age terribly. According to one of the charts in the links above, a 23-year-old reliever sees, on average, an FIP increase of 1.00 by age 26 or so. By age 30, that increase is an average of 1.50. Considering that average FIPs have been in the 4.00s, this means that an average reliever that’s age 23 will be hilariously unplayable at 30. A reliever will need to have an FIP in the low 3.00s to be average by the time he’s 26, or in the mid-2.00s to be average by the time he’s 30. Some relievers will hang on, sure. But there’s no great reason to count on any given reliever “learning how to pitch” or anything like that.

Before the 2018 season, I tried to come at this topic a different way, focusing instead on experience rather than actual age. I found pretty much the same thing: players have much greater odds of getting worse rather than getting better with every subsequent year. You can find that work here:

What’s the use of knowing all this? Well, aside from general knowledge, these aging curves help to explain how aging factors into forecasting player performance. One common rule of thumb used these days is that a player will “stay the same” up until or through age 30, then decline by 0.5 WAR per full season until or through age 35, and then decline by 0.75 WAR per full season thereafter. That’s actually pretty sizable: a perfectly good 3 WAR player will become just average in his early 30s, a bench guy before his mid-30s, and should be not-quite-deserving of a roster spot a few years after that. Again, not all projection systems will use similar assumptions, and this is a very rough, average way of thinking about aging in player forecasting. But, we know aging happens, so ignoring it seems unwise. When looking multiple years into the future, which happens most often when evaluating trades or considering potential free agent contracts, aging matters quite a bit.

tl;dr takeaway for aging - player primes are no longer late 20s. Players tend to decline at/after 30 years of age, and decline is pretty constant until they’re out of baseball. Pitchers also decline, and before they hit 30, their command tends to improve while velocity is always declining, which cancel each other out. In general, don’t assume that players will get better once they’ve debuted, because most don’t.